At the start of a block

A node is at the start of a block of content if one of the following is true:

In other words, the node is either the first perceivable content inside the block, any non-perceivable content before the block but after the previous perceivable content, or any non-perceivable content inside the block but before the first perceivable content in it.

Several nodes may be at the start of a block, especially if there are several non-perceivable content nodes next to each other.

Order ("first", "before", …) in this definition is tree order in the flat tree.